Earth
Earth is the third planet away from our Sun, and the only known planet to support life. The Earth holds many millions of species of life including seven billion humans as of 2014. Earth was formed around four and a half billion years ago, along with everything else in our system. At first the Earth was a molten rock of magma. Right before cooling down, a large celestial body collided with Earth and a chunk broke off, forming the Moon. Eventually, the planet began to evolve and an atmosphere and water were created. The atmosphere is like a shield that covers the world that has kept every specie safe from the Sun's harm. And while we only have one natural satellite, humans have sent up thousands of artificial satellites and other space junk. 'Composition' Earth is a rocky planet. It's mostly composed of iron, oxygen, silicon, and magnesium. Earth's core is mostly composed of iron and nickel, which both make up about 95% of the Earth's core. According to many scientists, the crust, which is the outermost layer of rock on Earth, and the smallest layer, is made of half oxygen and half other substances. 'Structure' 'Interior' The internal structure of the Earth is composed of three different parts: *crust *mantle *core 'Crust' The crust is the outermost layer of the Earth. There are two different types of crust: thin oceanic crust that underlies the ocean basins and thicker continental crust that underlies the continents. These two different types of crust are made up of different types of rock. The thin oceanic crust is composed of primarily of basalt and the thicker continental crust is composed primarily of granite. The low density of the thick continental crust allows it to "float" in high relief on the much higher density mantle below. 'Mantle' Earth's mantle is thought to be composed mainly of olivine-rich rock. It has different temperatures at different depths. The temperature is lowest immediately beneath the crust and increases with depth. The highest temperatures occur where the mantle material is in contact with the heat-producing core. This steady increase of temperature with depth is known as the geothermal gradient. The geothermal gradient is responsible for different rock behaviors and the different rock behaviors are used to divide the mantle into two different zones. Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot and soft, but not molten. Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes. However, rocks in the lower mantle are soft and flow when subjected to forces instead of breaking. The lower limit of brittle behavior is the boundary between the upper and lower mantle. 'Core' Earth's Core is thought to be composed mainly of an iron and nickel alloy. This composition is assumed based upon calculations of its density and upon the fact that many meteorites are iron-nickel alloys. The core is earth's source of internal heat because it contains radioactive materials which release heat as they break down into more stable substances. The core is divided into two different zones. The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy. However, the inner core is a solid even though its temperature is higher than the outer core. Here, tremendous pressure, produced by the weight of the overlying rocks is strong enough to crowd the atoms tightly together and prevents the liquid state. 'Exterior' The Earth is not a perfect sphere. Due to it's rotation, the Earth has a slightly distorted shape. The rotational momentum tends to force the matter to bunch up in the middle. In the case of the Earth, this "middle" is the equator. The true shape of the Earth called an Oblate Spheroid. The term "Oblate" refers to it's slightly oblong appearance. The term "Spheroid" means that it is almost a sphere, but not quite. One of the most important things to remember about the Earth's shape is that it is only very slightly oblate. The diameter from the North Pole to the South Pole is approximately 12,714 km. The equatorial diameter is approximately 12,756 km. This is not a big difference, but it does make the Earth not quite a sphere. Category:Planet Category:Terrestrial Planets